1. Field of the Invention
The present invention relates to communication of a data signal over a power distribution system, and more particularly, to a use of an inductive coupler for coupling a data signal via a conductor in a power transmission cable.
2. Description of the Prior Art
Low voltage (LV) power lines within the confines of a home or business have been used as a medium for point to point or network communications using so called “carrier” systems in which a data signal is modulated onto a high frequency (HF) carrier and transmitted over the power lines. Internet access, which requires “last mile” connectivity between the Internet data trunk and each domicile, would greatly enhance the utility of such networks.
A medium voltage (MV) typically 4-66 kV is reduced to a low voltage (LV) typically 100-500 volts, through an MV-LV distribution transformer. A medium voltage power distribution grid feeds many homes and businesses via distribution transformers. If data is present on the medium voltage power grid, it would be desirable to couple broadband data streams from transformer substations to entire sections of a neighborhood, but the distribution transformers effectively block high frequency energy and thus block the data from getting to the LV drop lines.
In countries using nominal low voltages of 125 volts or less, such as in North America, drop lines from the distribution transformer to the electrical load in the home or business are usually kept shorter than about 50 meters, so as to minimize voltage drop across the lines and to preserve adequate voltage regulation. Typically, only one to ten homes or businesses are supplied from each distribution transformer. For such a small number of potential users, it is not economical to procure an expensive high data rate feed, such as fiber or T1, and couple it via power line communications devices to the low voltage side of the transformer. Accordingly, in order to exploit the medium voltage distribution grid as a data backhaul channel, a device is required to bypass the distribution transformer.
In a power distribution system, a high voltage (HV) typically 100-800 kV, is stepped-down to a medium voltage through an HV-MV step-down transformer at a transformer substation. The high frequency blocking characteristics of distribution transformers isolate the medium voltage power distribution grid from high frequency noise present on both the low voltage and the high voltage (HV) lines. The medium voltage grid is thus a relatively quiet medium, ideal for communicating high speed data as a data distribution system or “backhaul line.”
The above-mentioned transformers block practically all energy in the megahertz frequency range. In order to couple high frequency modulated data from the MV lines to the LV lines, a bypass device must be installed at each transformer site. Devices are presently available and used for low frequency, low data rate data coupling applications. Such applications are often termed Power Line Communications (PLC). These devices typically include a high voltage series coupling capacitor, which must withstand a Basic Impulse Loading (BIL) voltage, typically above 50 kV. Such devices are thus expensive, bulky, and have an impact on overall power grid reliability. Furthermore, in some cases, during their installation they require disconnecting power from the customers.
In countries having a nominal low voltage in the 100-120 volt range, such as Japan and the US, the number of distribution transformers is especially large. This is because the MV-LV distribution transformers are placed relatively close to the load to keep the feed resistance low. Low feed resistance is desired to maintain reasonable level of voltage regulation, that is, minimal variation in supply voltage with varying load currents. LV feed lines for distances much in excess of 50 meters would require impracticably thick wires.
For a data coupler to be effective, it must be considered in the context in which it operates in conjunction with the high frequency characteristics of the MV power lines and with other components connected to these lines, such as transformers, power factor correction capacitors, PLC coupling capacitors, and disconnect switches. These components operate at different voltages in different countries and regions. The operating voltage level has a direct impact on the geometry of the construction of medium voltage power devices and the terminal impedance of these devices at Megahertz frequencies. Other factors affecting high frequency signals on MV power lines include the geometry of the network, e.g., branching, the use of very low impedance underground cables that connect to high impedance overhead lines, and the possibility of a splitting of a network into sub-networks due to an actuation of a disconnect switch. Therefore, the suitability of an MV-LV coupler device must be considered in the context of the specific characteristics of the equipment used in each country and the MV voltage level.
Overhead transmission lines are characterized by two or mores wires run at essentially constant spacing, with air dielectric between them. Such lines have a characteristic impedance in the 300 to 500 ohms range, and very low loss. Coaxial underground cables comprise a center conductor surrounded by a dielectric, over which are wound neutral conductors. Such cables have a characteristic impedance in the range of 20 to 40 ohms, and display loss for Megahertz signals that may be as low as 2 dB per hundred-meter length, depending on the loss properties of the dielectric.
An MV-LV distribution transformer, whether designed for operation from single phase to neutral or from phase to phase in a three phase grid, has a primary winding on the MV side that appears as having an impedance in the 40 to 300 ohm range for frequencies above 10 MHz. Power factor correction capacitors have large nominal capacitance values (e.g. 0.05-1 uF), but their high frequency impedance is primarily determined by series inductance inherent in their construction. PLC coupling capacitors have lower nominal capacitances, for example, 2.2-10 nF, but may have high frequency impedances that are relatively low relative to the power cable's characteristic impedance. Any of the aforementioned devices may produce a resonance in the megahertz range, i.e., the imaginary part of a complex impedance becomes zero ohms, but the devices do not have high Q factors at these frequencies, and so the magnitude of the impedance typically does not approach zero for a series resonance or an extremely high value for a parallel resonance.
Another device used on MV grids, especially in Japan, is a remotely controlled three phase disconnect switch. When a data signal is transmitted over a phase line that passes through such a switch, continuity of the data needs to be maintained even when the phase line is opened through the switch.